A substrate processing system includes a processing chamber and a plasma source located external to the chamber. The optical sensor module includes a light emitting source and a light receiving detector for communicating with the substrate and providing an indication of the diffusion of light through the substrate. Pairwise similarity is then used for optionally searching, classifying, or constructing classification systems. The rocker arm is movably disposed on the bracket. The sheath can include any suitable barrier material, and can, for example, contain one or more metallic materials.